Freestanding Binary Graphene/LiMn2O4 Nanorod Cathode Electrodes for High Energy Storage Applications

Abstract

The transition from ‘fossil ‘economy to a greener and sustainable economy cannot be achieved without efficient energy storage systems. The recovery of energy from renewable sources such as solar or wind power has enormous potential to meet current and future energy needs and to lead to a better preservation of nature and the environment. In the United States for example, the combustion of fossil fuel results in more than 90% of the greenhouse gas emissions, which is also the main cause of global warming. This noticeable climate changes have urged major vehicles producers to develop zero-emission vehicles (electric or hybrid vehicles). To achieve the aim to develop a suitable energetic solutions for mobile and stationary applications, an efficient and low cost energy storage system is needed. The lithium-ion batteries with high energy density have long been effective solution to meet these demands. Nano structured electrode materials have shown improved electrochemical performance due to the large surface area which facilitates the electrode-electrolyte contact. Tailoring the electrode morphology in the forms of nanowires and nanorods are particularly attractive since they provide large surface area and efficient one-dimensional electron transport pathways and facile strain relaxation during battery charge and discharge. Spinel LiMn2O4 based cathode electrode materials are one of the most promising alternative cathode materials for Li-ion power batteries, which is due to the advantages such as low cost, good environmental compatibility, and good thermal stability. It has been reported that the charged LiMn2O4 material shows obviously higher thermal stability in the electrolyte at high temperature than LiCoO2 and LiNiO2. The good thermal stability of spinel LiMn2O4 is beneficial for its use in high power batteries. Although many reports revealed that the LiMn2O4 based electrodes offer a potentially attractive alternative to the presently commercialized LiCoO2, there are still some issues prohibiting LiMn2O4 from commercialization is its severe capacity and cycling performance fading during cycling. It is reported that both the particle size and its size distribution play important roles in the electrochemical performance. To overcome this problem, many researchers are focused on nanocomposite electrodes. Mats or so-called freestanding electrode that contains high concentrations of graphene nanosheets have the potential to form strong and light weight composite materials. The formation of intense surface films may increase the electrode impedance and even electrically isolate part of the active mass. In this study, LiMn2O4 nanorods were produced via cost effective and facile method by chemically converting a-MnO2 structure. a-MnO2 nanorods were produced via microwave hydrothermal synthesis method KMnO4 and manganese sulfate MnSO4.H2O as the starting precursors. LiMn2O4 nanorods were then chemically obtained by using a-MnO2 nanorods by a simple solid-state reaction. As-synthesized LiMn2O4 nanorods were then decorated between graphene nanosheets through a vacuum filtration process and freestanding cathode electrodes were obtained. Graphene/LiMn2O4 cathode electrode exhibited higher rate capability, specific capacity and cycle performance when compared with pristine LiMn2O4 electrode. This is due high surface area of LiMn2O4 nanoparticles and good electronic conductivity of graphene. The improved electrochemical performance of the graphene coated LiMn2O4 electrode was attributed to decreasing Mn dissolution into electrolyte. The improved electrochemical performance of the LiMn2O4/graphene nanocomposite makes it the promising cathode material for high-performance lithium-ion batteries. Keywords: LiM2O4 nanorods, graphene, freestanding electrodes, Li ion batteries.